North Korea has expressed its commitment for "complete denuclearisation" of the Korean peninsula and is not seeking conditions, South Korean President Moon Jae-in has said, as the United States vowed to maintain "maximum pressure" on Pyongyang.

Mr Moon said big-picture agreements about denuclearisation, establishing a peace regime and normalisation of relations between the two Koreas and the United States should not be difficult to reach through summits between the North and South, and between the North and the United States.

"I don't think denuclearisation has different meanings for South and North Korea," Mr Moon said.

He added: "The North is expressing a will for a complete denuclearisation.

"They have not attached any conditions that the US cannot accept, such as the withdrawal of American troops from South Korea.

"All they are talking about is the end of hostile policies against North Korea, followed by a guarantee of security."

North Korea has defended its nuclear and missile programmes, which it pursues in defiance of UN Security Council resolutions, as a necessary deterrent against perceived US hostility.

The United States stations 28,500 troops in South Korea, a legacy of the 1950-53 Korean War.

North Korea has said over the years that it could consider giving up its nuclear arsenal if the United States removed its troops from South Korea and withdrew its so-called nuclear umbrella of deterrence from South Korea and Japan.

South Korea announced on Wednesday that it is considering how to change a decades-old armistice with North Korea into a peace agreement as it prepares for the North-South summit this month.

North Korea and South Korea are technically still at war because the 1950-53 conflict ended in a truce, not a peace treaty.

Mr Moon said he saw the possibility of a peace agreement, or even international aid for the North's economy, if it denuclearises.

But he also said the inter-Korean summit had "a lot of constraints", in that the two Koreas could not make progress separate from the North Korea-United States summit, and could not reach an agreement that transcends international sanctions.

"So first, the South-North Korean summit must make a good beginning, and the dialogue between the two Koreas likely must continue after we see the results of the North Korea-United States summit," Mr Moon said.

US CIA Director Mike Pompeo visited North Korea last week and met leader Kim Jong-un with whom he formed a "good relationship", US President Donald Trump said on Wednesday, ahead of a summit planned for May or June.

Mr Trump said he hoped the summit would be successful, but warned he would call it off if he did not think it would produce results.

Mr Trump told a joint news conference with Japanese Prime Minister Shinzo Abe that his campaign of "maximum pressure" on North Korea would continue until Pyongyang gave up its nuclear weapons.

"The United States remains committed to complete, verifiable and irreversible denuclearisation of North Korea," US Disarmament Ambassador Robert Wood told a news conference in Geneva on Thursday ahead of a two-week conference on the Nuclear Non-Proliferation Treaty.

"In terms of the pressure campaign, the things we have been very interested in are maintaining the pressure, meaning enforcing sanctions, ensuring that the North is not able to get access to funds that help further its nuclear and ballistic missile programmes."

North Korea must show that it is "serious about getting ridof its nuclear weapons programme" and take "concrete steps", Mr Wood said, adding: "But we've got a long way to go". 

Ahead of next week's summit, Seoul and Pyongyang will also complete the installment of a telephone hotline between the two leaders, directly connecting the South's presidential Blue House and the North's State Affairs Commission, the South's presidential spokesman said.

Six top South Korean officials will accompany Mr Moon to the summit, including his chief of staff, spy chief, national security adviser and unification, defence and foreign ministers, the spokesman said.

North Korea meanwhile will hold a plenary meeting of its ruling party's central committee on Friday, state media KCNA said.